Ethylene-vinyl alcohol copolymers (hereinafter, may be abbreviated as “EVOH(s)”) have been widely used as a material that is moldable through melting and superior in gas barrier properties. For example, EVOHs have been used as a material for films and sheets to be formed by melt molding. EVOH layers constituted with the sheets and the like have been used as a packaging material after being laminated on a thermoplastic resin layer containing an olefin-derived resin, etc. as a principal component. Such a packaging material having the EVOH layer may be subjected to thermoforming and thereafter utilized as a packaging container. Since the packaging container is superior in oxygen barrier properties due to including the EVOH layer, the packaging container has been widely used in intended usages in which superior oxygen barrier properties are demanded in a variety of fields of, for example, foods, cosmetics, medical drugs, chemicals, and toiletries.
However, the EVOH layer is generally inferior in thermoformability as compared with the thermoplastic resin layer containing the olefin-derived resin, etc. as a principal component. Thus, according to the packaging material including the EVOH layer, defects such as pinholes and cracks are likely to be generated during the thermoforming, and consequently the packaging container is likely to have an unfavorable appearance. Furthermore, the packaging material including the EVOH layer is likely to have unevenness in thickness after the thermoforming, and accordingly tends to cause drawbacks such as impairment of gas barrier properties and mechanical strength of a packaging container.
In order to overcome such drawbacks, addition of a plasticizer to a resin composition for forming a sheet and the like containing the EVOH as a principal component (see Japanese Unexamined Patent Application, Publication Nos. S53-088067 and S59-020345), blending of polyamide with such a resin composition (see Japanese Unexamined Patent Application, Publication No. S52-141785), and the like have been studied. However, all of these result in significant impairment of the gas barrier properties.
In addition, a production method of EVOH has been known in which crotonaldehyde is coexisted in addition to ethylene and vinyl acetate in a polymerization step to produce the EVOH (see Japanese Unexamined Patent Application, Publication No. 2007-31725). According to this production method, an adhesion of scales in the interior of a polymerization tank can be inhibited by the coexistence of crotonaldehyde during the polymerization. As a result, according to films of the EVOH produced by this production method, generation of fish eyes resulting from the scales detached and mixed into the polymer can be reportedly decreased.
However, crotonaldehyde added in the polymerization is partially consumed in the polymerization step and a saponification step. In addition, crotonaldehyde has a solubility in water of as high as 18.1 g/100 g (20° C.) (The MERCK INDEX 14th 2006). On the other hand, the production method of EVOH typically includes the step of washing with water away sodium acetate produced in the neutralization after the saponification. Thus, crotonaldehyde added in the polymerization is substantially completely eliminated in the washing step during the production of EVOH, and thus hardly remains in a product such as an EVOH film. Therefore, according to the production method described above, the effects of the addition of an unsaturated aldehyde, e.g., an improvement of thermal stability and properties for operation for a long time period (long-run workability) in the thermoforming, and the like are unknown.